The SIP framing is done. I thought (like a dummy) it would take less than a day, but it took over two weeks.

2 weeks of SIP assembly

The Process

A bunch of friends were scheduled to come in from NYC on the weekend of 7/22 to help construct the SIP frame, so I budgeted two days before to prep the trailer, build the floor, and pre-cut all the lumber for connecting the SIPs. I figured with that stuff ready, we’d get the walls and roof up in no time – I went so far as making a list of other things I could do with the extra hands and extra time. But by Fri 7/21 evening I had only assembled one of the floor panels. It was a pretty clear sign I had underestimated how long this would take.

We worked continuously through the weekend, but by the end we were nowhere near finished: we assembled the floor on the trailer (we hadn’t even bolted it down). Luckily I accepted that my prior expectations were outrageous before my friends arrived, so I was mostly able to relax and appreciate all the help.

We made a lot of less obvious progress and overcame many obstacles I hadn’t realistically planned for (most of these apply to the whole process, not just that weekend):

• Mistakes: None of us had SIP experience, so we made a lot of mistakes. Multiple times we stepped back and discovered something was not quite right: we had installed the lumber splines slightly out of position because the wood was warped (later we learned to force it into position with clamps), or in a sequence that would block access to fasten it to an adjoining panel (later we learned to think a few joins ahead), or some other thing. That meant pulling out the nails/screws, cleaning out the caulk, cutting new lumber, and starting over. This was aggravating at the time, but now that it’s over and things fit nicely, it was worth it.
• Rain: We were repeatedly hit by rain showers and had to stop work, cover the in-progress floor, restack and cover the raw materials, etc. During these showers we took what we could into the garage and continued working.
• Cutting Lumber: The SIPs are joined to one another with 2x lumber, and I really underestimated how much time was required to cut those – especially since many of them needed beveled cuts to mate with the angled shed roof and wheel wells. Though we didn’t get the satisfaction of installing them, we did measure, mark, label, and cut a good portion of the lumber in the garage during the periods of rain.
• Moving big things: Because the panels are bulky and we had no heavy equipment, moving them around (flipping to access the other side, re-stacking under a tarp for rain, etc.) took a surprising amount of time. And lifting the large walls and roof panels safely without cranes/forklifts required some creative and time-consuming solutions.

Cut and labeled lumber

After the crew left, I lost one day of construction to procuring standing-seam roof materials, then we got back to work. Over the next two weeks:

1. Affixed the SIP floor by bolting the sill plates down to the trailer, sandwiching the floor panels securely in place.
2. Raised the walls, starting with the back wall, then the back corners (which gives the free-standing walls much more stability), then the left and right walls.
3. Assembled the roof panels and raised them up with a system of ramps, pulleys, and stop blocks.
4. Installed the front wall, completing all walls and gaining the strength of 4 corners.
5. Fastened the roof down to the wall top plates.

6-inch structural screws with washers fasten one wall to another at the corners

SketchUp Model vs. The Real World

One of my main worries was that I would make some error measuring the trailer, and then all my SIPs would be precisely manufactured to the wrong size or shape and we’d need to spend days adjusting the panels to fit the trailer. Everything was cut exactly to my specifications, and luckily almost everything fit perfectly (we just needed to shave off 1/8” over one wheel well’s angle).

There were 2 things that differed between my 3D model and the real world:

1. We couldn’t always get each panel to mate flush with its neighbor’s edge. Due to warps in the connecting lumber splines there might be a 1/16” gap here, 1/8” there, which added to about 1/2” of extra length over the 19’ multi-panel walls . Since I had previously worked entirely in perfect 3D model land, it was pretty hard to accept the reality of the real world. But it was too late to start over, and I didn’t want to burn the whole thing down, so we shaved the extra off the front of the overhang and things were peachy again.
2. The floor has a slight dip in its back few feet that we didn’t detect with our level (0.3° off level, to be exact – calculated with a plumb line and some trig). I don’t know if this unlevel area is due to the trailer frame, my floor spacers (probably), or imperfections in the SIP floor assembly, but it’s there and we didn’t notice until after we installed the back wall and corners. If we had just installed all the wall panels flush with each other out from the back corners the front of the side walls would end up elevated by over an inch. To compensate, we butted the bottom corners of all the wall panels together as normal but left a slight gap at the top of each join to gradually come back to level over the length of the wall.

A temporary tarp roof saved us from some serious storms

Conclusion

I dunno. The jury is still out on whether SIPs were a good idea. The strength, insulation, and airtightness benefits are yet to be tested, but I think will make it all worthwhile over the long term. I suspect the construction speed benefit I was hoping for is enjoyed by professional crews with experience installing SIPs, but as a crew of noobs with no nail gun or heavy equipment perhaps traditional framing could have been faster. I can’t be sure of that either, as I haven’t done a traditionally-framed tiny house.

Anyway, it’s done, feels solid, and looks good!

The T.H.O.W. frame (not actually done here, but it’s the closest to done I photographed since we finished in the dark and tyveked everything the next morning.)